1. Field of the Invention
The present invention relates to a method of forming a rigid film of a calcium phosphate compound having good bio-conformance on a surface of a ceramic or metal substrate and, more particularly, to a method of forming the film by using a calcium phosphate compound slurry.
2. Description of the Prior Art
A single-crystal sapphire, a ceramic material (e.g., an alumina or zirconia sintered body) and a metal material (e.g., titanium, a titanium alloy, or a stainless steel) have high strength and are nontoxic to a living body. Such a material is used as an intracorporeal indwelling material such as an artificial dental root and an artificial bone. However, the material is inert with regard to the tissue of a living body and does not have a bonding capacity with a new bone. In addition, a wrapping phenomenon by a film of foreign matter tends to occur. As a result, the material cannot be embedded in a living body for a long period of time.
However, a calcium phosphate compound such as hydroxy apatite or tricalcium phosphate is a major constituent of the inorganic component in a living body such as a tooth and a bone and is not toxic to a living body. Therefore, the calcium phosphate compound has good bio-conformance such as a good bonding property with a bone and substitution with a new bone. However, a high strength sintered body of the calcium phosphate compound has never been proposed in practice.
In consideration of the above situation, a strong demand has arisen for an intracorporeal indwelling material having high strength and good bio-conformance. In order to meet such a demand, a composite material obtained by coating a calcium phosphate compound on a surface of a single-crystal sapphire, ceramic or metal material is most promising, and extensive studies on it have been made.
Conventional methods of forming a calcium phosphate compound film on a surface of an intracorporeal indwelling material are proposed as follows.
First, a calcium phosphate compound is melted by a high-temperature flame and sprayed at high speed on the surface of a substrate (spraying process).
Second, a calcium phosphate compound is sputtered on the surface of a substrate (sputtering method).
Third, a calcium phosphate compound slurry is applied to the surface of a substrate by spraying or dipping and is dried and sintered (slurry method).
Fourth, a chemical precipitation method proposed by the present applicant (Japanese Patent Disclosure (Kokai) No. 62-116781) is available.
The above conventional methods have both advantages and disadvantages. Improvements of these methods have been made. Of these conventional methods, the slurry method does not require special equipment and can be easily practiced at low cost. Therefore, the slurry method is most promising.
The slurry method, however, has disadvantages in that adhesion strength of the film with the substrate is low, and formation of a uniform film on the surface of the substrate is difficult due to the following reason. The particles of the calcium phosphate compound used to prepare a slurry have a large size of about 8 .mu.m. When the particle size is large, a coagulation force is generated between the particles, and the particles cannot be uniformly dispersed in the slurry. Therefore, the slurry cannot be properly applied to projections or recesses of the substrate, resulting in a nonuniform film. In addition, good sintering of the substrate and the film cannot be performed. The resultant film is nonuniform, and the film tends to peel from the substrate due to a difference between the thermal expansion coefficients of the film and the substrate.